Nitric oxide (NO) is a messenger molecule that plays an important physiological role, both intracellular and intercellular, in anti-platelet aggregation and anti-platelet activation, vascular relaxation, neurotransmission, and immune response. It has been proposed that synthetic materials that release low levels of NO or possess NO precursors on their surface would therefore more closely simulate the natural activity of endothelial cells, and therefore would improve tissue healing and the formation of vascularized tissue in proximity to an implant.
Nitric oxide (NO), a simple diatomic molecule, is a powerful signaling compound that plays a diverse and complex role in cellular physiology. NO is associated with endothelial cells, neural cells and macrophages. Mammalian cells synthesize NO using a two-step enzymatic process that oxidizes L-arginine to N-.omega.-hydroxy-L-arginine, which is then converted into L-citrulline and an uncharged NO free radical. Three different nitric oxide synthase enzymes regulate NO production. Neuronal nitric oxide synthase (NOSI, or nNOS) is formed within neuronal tissue and plays an essential role in neurotransmission. Endothelial nitric oxide synthase (NOS3 or eNOS), is secreted by endothelial cells and induces vasodilatation. Inducible nitric oxide synthase (NOS2 or iNOS) is principally found in macrophages, hepatocytes and chondrocytes and is associated with immune cytotoxicity.
NOS and eNOS are enzymes that regulate the release of small amounts of NO. NO activates guanylate cyclase which elevates cyclic guanosine monophosphate (cGMP) concentrations which in turn increase intracellular Ca+2 levels. Increased intracellular Ca+2 concentrations result in smooth muscle relaxation which accounts for NO's vasodilating effects.
Biological disorders associated with the implantation of a medical device, for example excessive fibrotic encapsulation, can be prophylactically ameliorated by supplying the repair site with therapeutic levels of NO. This can be accomplished by stimulating the endogenous production of NO or using exogenous NO sources.
Regarding stimulation of endogenous NO formation, nitric oxide synthases (NOS) produce NO by replacing a N═C double bond with an O═C double bond. Researchers have focused on activation of enzymatic pathways with excess NO metabolic precursors like L-arginine and L-lysine. Therefore, there is interest in a metabolic precursor which may not itself release NO but may catalyze endogenous NO release. Synthetic precursors that release NO are of interest in this connection.
The exogenous administration of gaseous nitric oxide is generally not feasible due to the highly toxic, short-lived, and relatively insoluble nature of NO in physiological fluids. As a result, the clinical use of gaseous NO is largely restricted to the treatment of neonates with conditions such as persistent pulmonary hypertension. Alternatively, however, the systemic delivery of exogenous NO with such precursor drugs as nitroglycerin has long enjoyed widespread use in the medical management of angina pectoris associated with atherosclerotic narrowing of coronary arteries. There are problems with the use of agents such as nitroglycerin. Nitroglycerin requires a variety of enzymes and cofactors in order to release NO, repeated use of this agent over short intervals produces a diminishing therapeutic benefit. By contrast, if too much nitroglycerin is initially given to the patient, it can have devastating side effects including severe hypotension and free radical cell damage.
One potential method for overcoming the disadvantages associated with NO precursor drug administration is to provide NO-releasing molecules that do not require activation by endogenous enzyme systems.
NO and precursors of NO are typically unstable, and are too reactive to be used without some means of stabilizing the molecule until it reaches the treatment site. NO can be delivered to treatment sites in an individual by means of polymers and small molecules which release NO. However, these polymers and small molecules typically release NO rapidly. As a result, they have short shelf lives and rapidly lose their ability to deliver NO under physiological conditions. For example, the lifetime of S-nitroso-D,L-penicillamine and S-nitrosocysteine in physiological solution is no more than about an hour. As a result of the rapid rate of NO release by these compositions, it is difficult to deliver sufficient quantities of NO to a treatment site for extended periods of time or to control the amount of NO delivered. Additionally, rapid release of NO and consequently high concentrations of NO is not always beneficial, especially in a healing promotion modality where low concentrations are required throughout the healing period, which can last up to six months.
Nitric oxide and nitric oxide donor compounds have been used for treating cardiovascular diseases, hypertension, inflammation, pain, fever, gastrointestinal disorders, ophthalmic diseases, glaucoma, ocular hypertension, hepatic disorders, renal diseases, nephropathies, diabetes, respiratory disorders, immunological diseases, bone metabolism dysfunctions, central and peripheral nervous system diseases, sexual dysfunctions, infectious diseases, for the inhibition of platelet aggregation and platelet adhesion, for treating pathological conditions resulting from abnormal cell proliferation, vascular diseases, neurodegenerative disorders, metabolic syndrome, Reynolds' syndrome, scleroderma, muscular dystrophies such as Duchenne and Becker dystrophies.
These treatment successes would be enhanced by providing a compound that supports endogenous NO formation or NO release that is slow and of long duration. Accordingly, the therapeutic compound must be stabilized against rapid degradation and elimination from the body. This requires the NO precursor be stabilized on the delivery molecule and the delivery molecule itself be of sufficient size or bonded to an implant to mitigate its early elimination from the body.
A very important class of NO precursor agents is the nitric oxide-nucleophile complexes. Methods for treating cardiovascular disorders in a mammal with certain nitric oxide-nucleophile complexes have been disclosed. These compounds contain an anionic N.sub.2O.sub.2.sup.-group or derivatives in a diazeniumdiolate. Many of these compounds have proven especially promising pharmacologically because, unlike nitrovasodilators such as nitroprusside and nitroglycerin, they release nitric oxide without first having to be activated. The only other series of drugs currently known to be capable of releasing nitric oxide purely spontaneously is the S-nitrosothiol series, compounds of structure R—S—NO. There are manufacturing complications associated with the synthesis of R—S—NO complexes. Nevertheless, these structures tend to degrade rapidly in vivo and are unstable to ambient conditions in storage.
While N-based diazeniumdiolate polymers have the advantages of localized spontaneous release of NO under physiological conditions, a major disadvantage associated with all N-based diazeniumdiolates is their potential to form carcinogenic nitrosamines upon decomposition. Some nitrosamines are extremely carcinogenic and the potential for nitrosamine formation limits the N-based diazeniumdiolate class of NO donors from consideration as therapeutic agents based on safety issues.
Therefore, if these structures are to be used, there is a need to make the byproducts of these structures more biocompatible. For example, there is a need to both stabilize the NO-releasing moiety in vivo, and to render the resulting molecule more biocompatible.
Currently, NO generation is determined by water uptake (such as in the case of diazeniumdiolates) or the intensity of light (as with iron nitrosyls). However, blood already contains a host of species that are derived from, or are physiologically-generated in vivo that may be reduced to NO. These species include nitrites, nitrates, and a host of nitrosothiols (e.g. nitrosoglutathione, nitroso albumin, etc.). The presence of these species raises the possibility of recycling these species back to nitric oxide in the presence of a synthetic molecule. Thus, there is a need for a synthetic molecule that acts synergistically with in vivo constituents that reduce to NO.
Alternatively, to treat a disorder with nitric oxide over a period of time, two compounds could be co-administered-one compound with a quick release of NO and a second compound with an NO release rate several times longer than the first compound. Unfortunately, at present, a suitable long term NO release moiety is not commercially available.
As another alternative, the same compound could be administered multiple times in order to provide a lasting treatment. However, this method increases the cost of treatment because of increased dosing and subjects the patient to increased exposure to any potential side effects.
The most promising NO precursors are those with a cyclic polyamine structure. For example, adducts of cyclic polyamine piperazine with nitric oxide have been studied. The bisdiazeniumdiolate of piperazine has been reported to have a biphasic release of NO with an initial half-life of 2.3 minutes and a secondary half-life of 5.0 minutes. In fact due to the similarity in the initial and secondary release rates for the bisdiazeniumdiolate of piperazine, it was initially believed that the two release rates were identical. Because the profile of the biphasic release of NO from the bisdiazeniumdiolate of piperazine was on such a similar time scale, and because the second half-life of NO release was only 5.0 minutes, such a compound is not practical for use in implant situations where tissue healing around the implant is desired. In addition, the use of piperazine diazeniumdiolate in pharmaceutical compositions is not desirable because of the potential toxicity of its possible nitrosopiperazine metabolite.
NO has also been found effective against infection. NO may contribute to the morbidity of infection by acting as a vasodilator, myocardial depressant, and cytotoxic mediator. On the other hand, microvascular, cytoprotective, immunoregulatory, and antimicrobial properties of NO have a salutary and probably essential role in the infected host. However, in the context of NO-releasing synthetic molecules, there is potential for direct elimination of a pathogen. Yet, to date direct elimination of a pathogen by NO release has not been demonstrated.
In the context of antimicrobial activity, most antimicrobials are cytotoxic, and at a minimum interfere with or reduce healing in the content of prosthetic implantation. Thus, there is a need for a bioactive agent that both enhances the healing response and directly eliminates microbes. The potential of NO to up-regulate vascularization in healing, supply more blood flow to a repair site, and diminish the incidence of exogenous and endogenous microbial colonization of a repair site or prosthetic is of clinical interest.
In recognizing the aforementioned aspects of the current state of the art, the following patents and applications are relevant in the present context.
U.S. Pat. Nos. 5,155,137 and 5,250,550 describe complexes of nitric oxide and polyamines which are useful in treating cardiovascular disorders, including hypertension. The disclosed compounds release nitric oxide (endothelium-derived relaxing factor) under physiological conditions in a sustained and controllable fashion, and possess long mechanisms of action.U.S. Pat. Nos. 5,366,997 and 5,405,919 describe oxygen substituted derivatives of nucleophile-nitric oxide adducts as nitric oxide donor prodrugs.U.S. Pat. Nos. 5,525,357 and 5,650,447 describe a polymeric composition capable of releasing nitric oxide including a polymer and a nitric oxide-releasing N.sub.2 O.sub.2.sup.-functional group bound to the polymer; pharmaceutical compositions including the polymeric composition.U.S. Pat. Nos. 7,087,709 and 7,417,109 describe novel polymers derivatized with at least one —NO.sub.x group per 1200 atomic mass unit of the polymer. X is one or two. In one embodiment, the polymer is an S-nitrosylated polymer and is prepared by reacting a polythiolated polymer with a nitrosylating agent under conditions suitable for nitrosylating free thiol groups.U.S. Pat. No. 7,226,586 describes extremely hydrophobic nitric oxide (NO) releasing polymers. The extremely hydrophobic NO-releasing polymers provided are extensively cross-linked polyamine-derivatized divinylbenzene diazeniumdiolates.U.S. Pat. No. 7,425,218 describes an implant or intravascular stent comprising a polymeric composition capable of releasing nitric oxide under physiological conditions.U.S. Pat. No. 7,569,559 describes compositions comprising carbon-based diazeniumdiolates that release nitric oxide (NO). The carbon-based diazeniumdiolated molecules release NO spontaneously under physiological conditions without subsequent nitrosamine formation.U.S. Pat. No. 7,763,283 describes biocompatible materials that have the ability to release nitric oxide (NO) in situ at the surface-blood interface when in contact with blood. The materials which may be polymers (e.g., polyurethane, poly(vinyl chloride), silicone rubbers), metals, such as stainless steel, carbon, and the like are provided with biocatalysts or biomimetic catalysts on their surface that have nitrite, nitrate, and/or nitrosothiol-reducing capability.U.S. Pat. No. 7,811,600 describes implantable medical devices comprising nitric oxide (NO) donating polymers comprising polymer backbones having at least one cyclic amine disposed thereon.U.S. Pat. No. 7,829,553 describes compositions comprising carbon-based diazeniumdiolates attached to hydrophobic polymers that releases nitric oxide (NO).U.S. Pat. No. 7,928,079 describes compounds capable of releasing nitric oxide wherein the compounds comprise a saccharide and at least one nitric oxide-releasing diazeniumdiolate [N.sub.2O.sub.2] functional group, which is bonded directly to a carbon atom of the saccharide, and methods for preparing the same.U.S. Pat. No. 7,928,096 describes polydiazeniumdiolated cyclic polyamines with polyphasic nitric oxide release and related compounds, compositions comprising same and methods of using same.U.S. Pat. No. 7,968,664 describes novel nitric oxide-releasing polymers that comprise at least two adjacent units derived from acrylonitrile monomer units and containing at least one carbon-bound diazeniumdiolate.U.S. Pat. No. 8,003,811 describes nitric oxide donors and pharmaceutically acceptable salts or stereoisomers.U.S. Pat. No. 8,021,679 describes implantable medical devices and/or coatings comprise NO-releasing biodegradable polymers derived from [1,4]oxazepan-7-one and its derivatives.U.S. Pat. No. 8,034,384 describes a material including a surface and a reactive agent that is located at the surface of the material, covalently attached to a backbone of the material, and/or located within the material. The reactive agent has nitrite reductase activity, nitrate reductase activity, and/or nitrosothiol reductase activity.U.S. App. No. 20100303891 describes a bio-adhesive supramacromolecular complex containing an NO releasing group.U.S. App. No. 20110117164 describes a method for increasing, prolonging, and/or controlling the release rates of nitric oxide (NO) from polymeric materials containing NO adducts.
Despite the promise of the nitric oxide/nucleophile adducts that have been investigated, their implantable applications are limited by their tendency to disassociate from the implant or repair site and distribute systemically. Distribution systemically tends to compromise the local benefit of being implanted along with an implant or at a surgical repair site. Thus there remains a need for nitric oxide-releasing compositions which are capable of concentrating the effect of the nitric oxide release to a situs of application and for which nitric oxide release may be controlled for effective dosing.